An objective lens plays a role of converging laser light and forming a small light spot on an information recording surface of an information recording medium such as an optical disc. There is widely used a technique of combining a refractive lens and a diffraction structure in order to implement a compatible lens for converging laser light through a single objective lens with respect to optical discs having different protective substrate thicknesses such as a compact disc (CD), a digital versatile disc (DVD), and a Blu-ray disc (BD) (see e.g. patent literature 1).
It is known that an objective lens provided with a diffraction structure (a step structure having a step for giving an optical path length difference with respect to an incident light flux) generates diffracted light (normal diffracted order light) of normal order which is converged on an information recording surface of an optical disc, and generates diffracted light (unwanted diffracted order light) of unwanted order which is not converged on the information recording surface of the optical disc.
In view of the above, patent literature 2 discloses an optical head (an optical information recording/reproducing device) provided with an objective lens provided with a step structure having a step for giving an optical path length difference with respect to an incident light flux, and configured to generate normal diffracted order light which is converged on an information recording surface of an optical disc, and unwanted diffracted order light which is collected at a position away from the information recording surface of the optical disc, wherein the distance from a convergent position of normal diffracted order light to a collecting position of unwanted diffracted order light is twice or more of the pull-in range of a focus error signal to be obtained in recording or reproducing information with respect to the optical disc.
Patent literature 2 pays attention to a focus error signal in applying an objective lens provided with a diffraction structure (a step structure) to an optical head. In patent literature 2, in recording or reproducing information on or from an information recording surface of an optical disc, the zero-cross point of a focus error signal based on unwanted diffracted order light is greatly away from the zero-cross point of a focus error signal based on normal diffracted order light. Thus, patent literature 2 tries to maintain the waveform of a focus error signal to have an S-shape necessary for effectively implementing the focus servo function.
FIG. 14 is a diagram for describing the characteristics of a focus error signal disclosed in the conventional art. Referring to FIG. 14, the vertical axis indicates an intensity of a detected focus error signal, and the horizontal axis indicates a moving amount of an objective lens in the optical axis direction. The right side of FIG. 14 corresponds to a direction in which the objective lens is moved toward an optical disc, and the left side of FIG. 14 corresponds to a direction in which the objective lens is moved away from the optical disc.
Patent literature 2 is configured such that a collecting position of unwanted diffracted order light is greatly away from an information recording surface of an optical disc (in other words, a convergent position of normal diffracted order light). More specifically, the distance from a convergent position of normal diffracted order light to a collecting position of unwanted diffracted order light is configured to be twice or more of the pull-in range of a focus error signal to be obtained in recording or reproducing information with respect to an optical disc. The pull-in range of a focus error signal indicates a range in which the focus servo function is effectively implemented by driving an actuator in accordance with a control voltage, and corresponds to a linear range of a focus error signal based on normal diffracted order light illustrated in FIG. 14.
Patent literature 2 describes that the above configuration makes it possible to secure a large distance between the zero-cross point of a second focus error signal based on unwanted diffracted order light and the zero-cross point of a first focus error signal based on normal diffracted order light in recording or reproducing information on or from an information recording surface of an optical disc, and thereby, it is possible to maintain the waveform of the first focus error signal based on normal diffracted order light to have an S-shape necessary for effectively implementing the focus servo function.
Actually, however, as will be described later, in the case where an objective lens provided with a diffraction structure is applied to an optical head, a third focus error signal is generated between a first focus error signal based on normal diffracted order light and a second focus error signal based on unwanted diffracted order light. As a result, only with the configuration such that the distance from an information recording surface of an optical disc (in other words, a convergent position of normal diffracted order light) to a collecting position of unwanted diffracted order light is twice or more of the pull-in range of a focus error signal, the zero-cross point of the first focus error signal based on normal diffracted order light and the zero-cross point of the third focus error signal come close to each other, and it is impossible to maintain the waveform of the first focus error signal based on normal diffracted order light to have an S-shape necessary for effectively implementing the focus servo function.
As described above, patent literature 2 has no consideration about a third focus error signal which may appear at an intermediate position between a first focus error signal based on normal diffracted order light and a second focus error signal based on unwanted diffracted order light, and the focus servo function may not be effectively implemented.